1. Field of the Invention
The present invention relates to a refrigeration system. More specifically, the present invention relates to a compound refrigeration system used to air condition a building or the like.
2. Description of the Related Art
Due to existing utility rate structures, electricity is most expensive during periods of peak demand, e.g., during daylight hours, and least expensive during periods of low demand, e.g., during nighttime hours. Daylight hours also typically are the time of highest air temperatures and human activity. Consequently, peak air conditioning loads usually occur in the afternoon, at the time that energy use is highest and electricity is most expensive. Minimum air conditioning loads typically occur at night, when energy demand is lowest and electricity is least expensive.
Thermal storage systems have been proposed for air conditioning spaces. The concept of thermal storage is to generate a cooling potential in the evening hours when energy is least expensive, store the cooling potential, and use the cooling potential to cool air during daylight hours, when energy is most expensive.
Conventional thermal storage systems include a single compressor, a single chiller, and a condenser. The compressor compresses a refrigerant, e.g., freon, which gives up its heat in the condenser, then cycles through the chiller. During daylight hours, water is flowed through the chiller to be chilled by the refrigerant. The chilled water then is used to cool the air, according to conventional chiller techniques. During nighttime hours, however, a brine solution, e.g., ethylene glycol or the like, is flowed through the chiller. The chilled brine is then used to freeze water into ice. This ice is then available to chill water during the following day.
Conventional thermal storage systems have drawbacks. The single compressor and chiller can be used only to chill water or to chill brine, not both. Consequently, if it is necessary to chill water for air conditioning purposes during the nighttime, a separate refrigeration/air conditioning system is often needed. This is inefficient and expensive.